In through-hole plating for the manufacture of a printed circuit board, the subtractive process is most frequently utilized. The subtractive process comprises drilling through-holes in a substrate consisting of an insulating board and a copper foil cladding disposed on either side thereof, causing a thin electroless copper plate to precipitate on the nonconductive walls defining the through-holes to render them electrically conductive, and depositing a thick copper electroplating layer to establish electrical connection between the copper foils and thereby complete an electric circuit.
This is a reliable technology well-established and use-tested over about a couple of decades. However, this process involves electroless plating as an essential step and this step requires a complicated pretreatment for rendering the nonconductive surfaces electrically conductive. The plating solution for use in electroless plating is so self-decomposing as to call for meticulous control over the bath composition. Moreover, as a reducing agent required for precipitation of copper, the use of formaldehyde which is a highly toxic material is essential. A further disadvantage of the process is that it takes a long time even to attain quite a thin copper layer.